Tuneable lasers are well known in the field of optical communications, particularly in connection with wavelength divisional multiplex telecommunication systems, which rely upon either being fed by stacks of individually wavelength distributed Bragg reflectors (DBR) lasers, which can be individually selected, or by a wide tuning range tuneable laser that can be electronically driven to provide the wavelength required. Limited tuning range tuneable lasers that rely upon thermal effects for tuning are also available.
U.S. Pat. No. 4,896,325 discloses a wavelength tuneable laser having sampled gratings at the front and rear of its gain region. The gratings produce slightly different reflection combs which provide feedback into the device. The gratings can be current tuned in wavelength with respect to each other. Co-incidence of a maximum from each of the front and rear gratings is referred to as a supermode. To switch the device between super modes requires a small electrical current into one of the gratings to cause a different pair of maxima to co-incide in the manner of a vemier. By applying different electrical currents to the two gratings, continuous tuning within a supermode can be achieved. In practice, the reflection spectra of the known sampled grating structures have a Gaussian type envelope which limits the total optical bandwidth over which the laser can reliably operate as a single mode device.
In contrast to the Segmented Grating Distributed Bragg Reflector (SG-DBR) described above, a Phase Shift Grating Distributed Bragg Reflector (PSG-DBR) is disclosed in GB 2337135. This has a plurality of repeat grating units in which each grating unit comprises a series of adjacent gratings having the same pitch, which gratings are separated by a phase change of π radians, wherein the gratings have different lengths to provide a pre-determined reflection spectrum.
The known devices have Bragg gratings which bound both ends of the gain and phase regions of a four section tuneable laser, which produces a comb wavelength response. For a given set of drive currents in the front and rear grating sections, there is simultaneous correspondence in reflection peak at only one wavelength, as a consequence of which the device lases at that wavelength. To change this wavelength a different current is applied to the front and rear gratings. Thus the front and rear gratings operate in a vernier mode, in which the wavelengths of correspondence determine a supermode wavelength. Although the known devices have generally been acceptable, they share a tendency to suffer from short wavelength losses, which in combination with the front grating tuning absorption reduces the output power of the laser.